The present invention relates generally to the energy conversion of sources of radiation, such as light. Specifically, the present invention provides a unique highly-efficient device and method for photoelectrolysis and artificial photosynthesis which needs no outside electrical energy source.
Electrolysis of various aqueous solutions to produce hydrogen, chlorine, bromine and the like, or to deposit various metals, has been known for many years. Generally, however, external sources of electrical energy are required for the process. Thus, prior methods of electrolysis are costly and inefficient due to the use of electrical energy which in most cases is produced by technologies involving a Carnot cycle.
The problem of high cost and low efficiency creates the need for an alternative non-carnot based extensive source of electrical energy. This problem can be solved by using light energy to decrease the need for expensive Carnot based external electricity, or to avoid the need completely. At present, four related light-based systems, having distinctive features, are possible. These are: (i) photovoltaic array and a separate water electrolyzer, giving rise to two plants; (ii) colloidal semiconductor systems which operate on solar energy alone as input; (iii) photo-aided electrolysis, necessitating both solar and electrical energies as inputs; and (iv) photoelectrolysis, requiring only solar energy as input.
The photovoltaic array system consists of a photoactivated semiconductor device, typically single crystal silicon, which when irradiated, produces an electric current. The current is applied to a conventional water electrolyzer. The need to connect several of the photovoltaic cells in series to obtain sufficient voltages for many electrochemical applications, increases the requirement for space and the cost of materials. In addition, a defective cell or broken electrical contact in such systems leads to significant energy losses. As a consequence, manufacturing standards and costs are raised.
Colloidal semiconductor systems consist of electrocatalyst coated submicron semiconductor particles suspended in an electrolyte solution and which operates on solar energy as input.
In photo-aided electrolysis systems either a p-type or n-type semiconductor electrode coupled to a metal oxide electrode and the like, or a p-type semiconductor electrode coupled to an n-type semiconductor elctrode, are immersed in an aqueous solution and the semconductor materials irradiated with light at the semiconductor/solution interface. However, an external source of electrical energy is needed in addition to light energy to drive the desired reaction. Thus, although the need for external electrical energy is reduced, the costly use of external electrical energy makes the devices less efficient than desired.
Photoelectrolysis is a system similar to photo-aided electrolysis except that no external electrical energy is required to drive the reaction. Photoelectrolysis systems, however, are typically limited in their application because of the relatively low solar energy conversion efficiencies currently obtainable.
Colloidal, photoelectrolysis and photo-aided electrolysis systems suffer from several disadvantages, including damage and inefficiency resulting from immersing semiconductors in the electrolyte, inefficient use of space and inefficient use of materials.
When semiconductors are immersed in the electrolyte, damaging photocorrosion or photopassivation phenomena generally results from interaction of the semiconductors with the intermediates and/or the products of electrochemical reactions. Of particular concern is the problem of hydrogen embrittlement where one of the electrochemical reactions involves hydrogen evolution. Hydrogen embrittlement involves the diffusion of adsorbed hydrogen species into the bulk of the semiconductor materials giving rise to localized highly stressed regions which promotes cracking and breakdown of the semiconductor material.
Efforts are generally made to protect semiconductors from these damaging interactions by coating the semiconductors with extremely thin layers of materials. The coatings are extremely thin in order to allow light to pass through to the semiconductor. The maximum thickness suitable for allowing sufficient light transmission is on the order of 40-100 angstroms. Typically the material coated on the semiconductor is a suitable catalyst for the particular electrochemical reaction desired. Unfortunately, because of their extremely thin nature these catalysts are also damaged and worn through photocorrosion. Additionally, the thin electrocatalyst layers generally have small pin-sized holes which allows the electrolyte to contact the semiconductor material. As a result, the protection afforded to semiconductors by these coatings are short lived, and the catalytic activity rapidly reduced. Accordingly, to avoid damage to semiconductors, and to maintain the electrical efficiency of the system, frequent replacement of typically expensive catalyst layers is required. However, frequent replacement of catalysts in industrial or household installations is impractical.
Immersing semiconductors in the electrolyte creates still other disadvantages. Since the electrical current created by the semiconductors is dependent upon the intensity of the light which reaches the semiconductors, any barriers to light transmission reduces the efficiency of the system. Light is in part reflected at the boundary between two transparent media, thereby reducing its intensity. This is the case in known light activated electrolysis devices, where light must pass through an aqueous electrolyte solution, and generally through a transparent catalyst layer before reaching the semiconductor. Light is lost due to reflection by the transparent material housing the semiconductor and the electrolyte, the electrolyte, the semitransparent catalyst, and in part by the semiconductor surface itself. In addition, light photons are absorbed by the electrolyte, further reducing the light intensity reaching the semiconductor. If the device can only generate sufficient voltage to split hydrogen bromide, the electrolyte solution may become colored as a result of the electrochemical oxidation of bromide ions thereby further decreasing light transmission.
In applications for producing hydrogen, the disadvantages of the above described devices have resulted in extremely low efficiencies of solar energy conversion to hydrogen, generally on the order of 1%. Moreover, for the efficient production of hydrogen, solutions have been limited to solutions containing hydrogen bromide because of the relatively low voltages supplied by previously known devices. Although only relatively small and easily obtainable voltages are required for the electrolysis of HBr, the bromine gas produced is an undesirable by-product of the reaction. Similarly in the case of the electrolysis of chloride-containing solutions the chlroine gas produced is an undesirable by-product as well. Because of their poisonous nature, these gases pose a potential hazard. In addition, these systems are generally closed, i.e., the hydrogen and bromine must be recombined in a fuel cell to give back the original hydrogen bromide, which can then be re-used as the electrolyte. A distinct disadvantage of the colloidal system is that evolved gases cannot be separated. If hydrogen and oxygen are evolved, dangerous explosive conditions may result. However, hydrogen is a highly desirable fuel in itself as well as a valuable chemical feedstock for the production of ammonia, methanol, synfuels and the like. Hydrogen removed from a cell may be stored for later use in a fuel cell, for use in an internal combustion engine or for industrial or household functions, such as heating, cooling or cooking.
For the above reasons, the electrolysis of water is highly desirable. Among other things, oxygen is easily vented to the atmosphere, thereby providing a beneficial effect. Working against these advantages, however, is the fact that known electrolysis devices which require no external sources of electricity, need at least four photo-activated semiconductor cells in series to produce sufficient voltage for the practical electrolysis of water (cell as used here means a semiconductor having n and p material). This results in the inefficient use of materials, space and available light energy.
A feature of the present invention is its ability to correct the inefficiencies encountered with previously known electrolysis devices. The semiconductor material is external to the electrolyte solution, thereby avoiding the problem of photocorrosion. Further, since the semiconductor material is external to the electrolyte, full advantage of available light energy may be obtained since light intensity is not decreased by semitransparent or translucent barriers. Catalysts, in addition, may be thicker since light need not pass through the catalysts to reach the semiconductor material. Consequently, the catalysts provide greater protection to underlying material as well as provide the extended catalytic activity required for a practical operating device. Another feature of the present invention is that by coupling the above gained advantages to the use of photo-activated semiconductors of suitable voltage output, the desirable advantage of electrolyzing water may be obtained using fewer semiconductors than previously required which results in the need for less space and maximizes the use of available light energy.